Electrical power cable systems often utilize dielectric insulating fluid as a means of preserving the integrity of electrical cable insulation constructed from dielectric fluid-impregnated electrical insulation materials and in some cases, for cooling of the cable. In a pipe-type electrical power transmission cable system, dielectric insulating fluid surrounds insulated conductors within a pipe. In a self-contained electrical power transmission cable system, dielectric insulating fluid is introduced into the cable system via one or more fluid passages constructed within the cable, and the fluid pressure is retained within the cable by the action of outer-non-permeable sheathing which surrounds one or more insulated conductors. As is known in the art, the fluid is maintained under pressure by one or more external sources of fluid under pressure, such as pumping stations or pressurized fluid reservoirs and can be static or circulated within the pipe or duct. When such pipe-type or self-contained cables are run from points of lower elevation to points of higher elevation, a pumping station at the lower elevation point can be used to pressurize or circulate the insulating fluid. However, the pumping station can be at the higher elevation point, or there can be several pumping stations located at one or more end points and/or at points intermediate to the lowest elevation and highest elevation points.
Most modern dielectric fluids are synthetic in nature, while many older installations contain oil-based dielectric fluids. Regardless of the specific type of dielectric fluid under consideration, nearly all environmental regulatory agencies concur in that the release of significant quantities of dielectric fluid into the environment is highly undesirable. In fact, various state and local environmental regulatory bodies have acted to restrict or eliminate the construction of new fluid-filled cable systems, particularly if these new systems have been designed without provision being made for the restriction of fluid loss from the cable system. Furthermore, many electrical utility companies who are possessed of large existing fluid filled cable systems, representing tremendous capital investment, have been recently presented with mandates requiring the modification of these existing cable systems to incorporate such fluid loss restrictive provisions, particularly in the case of cable systems crossing or lying near notable bodies of water. Breaks or ruptures in the pipe or cable sheath can cause the unrestricted release of thousands of gallons of fluid, which can cause substantial losses of plant and animal life. In addition to the environmental impact, replacement of the released fluid is expensive. In addition, once the fluid has escaped from the cable, water or dirt can enter the cable or cable system pipeline through the break. Such contamination of the cable or cable system pipeline by the environment can require the replacement of significant lengths of cable.
Although various types of electrical power transmission cables exist which do not require the presence of fluid dielectric material, these types of cables do not have the decades long history of reliable operation at high voltages and high ampacities as do fluid-filled cable systems. Also, the replacement of the hundreds of miles of existing fluid-filled cable systems, (in the United States mostly pipe-type cable systems), with solid dielectric insulation cables would involve such extraordinary cost as to be unfeasible. This solution to the environmental regulatory concerns stated previously becomes even more unattractive when it is considered that the lower current carrying capacity of solid dielectric insulation type cables would require that a given number of existing pipe-type cable circuits be replaced with a greater number of solid dielectric insulation type cable circuits, if the operating voltages are left unchanged.
Superconducting cables are being developed which are intended to serve as transmission cable systems. For retro-fit pipe installations and new installations (utilizing superconducting cables), the cables being developed will utilize dielectric fluid impregnated electrical insulation of a similar nature to that which is currently used as the electrical insulation for pipe-type cables. In fact, the construction of these cables, as presently envisioned will be such that existing copper conductor type and aluminum conductor type pipe-type cable systems can be retro fitted with the superconducting cable, leaving the pipe system manholes and pressure support equipment essentially unchanged. Obviously, with the operation of these superconductive cable systems remaining dependent on the use of fluid dielectric materials within the system pipeline, the same concerns regarding the restriction of dielectric fluid leakage from the pipeline apply.
To address these problems, well-known stop joints are typically provided between cable portions or sections to hydraulically isolate such cable portions or sections. The stop joint is a device which mechanically and electrically interconnects cable sections, but which prevents the flow of the fluid directly from the pipe of one pipe cable section to the pipe of the next pipe section, in the case of pipe cables, or from the fluid duct of one self-contained cable to the fluid duct of the next self-contained cable. However, each stop joint has manually operable by-pass valves and piping which interconnect one side of the stop joint with the other side of the stop joint so that when the by-pass valves are open, the fluid can flow therethrough and between the pipes or ducts of the cable sections connected to the joint. Such by-pass valves are usually accessed through a manhole.
In the event of a break in a pipe or duct of such a system and even though the break and reduction in pressure may be sensed at the pumping station, causing the pumping station to cease the supply of fluid, fluid in the pipe or duct at elevations above the rupture site, including fluid above the nearest stop joint, flows towards the rupture site due to gravity. Fluid from above the stop joint is lost through the rupture until the rupture is located and the nearest by-pass valve above the rupture is manually closed by maintenance crews, who must enter the appropriate manhole. Thousands of gallons of fluid can be lost before the proper valve is closed, causing severe damage to the environment and monetary loss. Once the valve is closed and the fluid flow ceases, the cable is exposed to contamination from the environment.
One alternative to the manual by-pass valve is to put sensors and a motorized valve in the manhole. This is not cost effective, however. It is also undesirable to provide electrical power in the manhole.
U.S. Pat. Nos. 5,207,243 and 5,280,131, both issued to Sarro, describe a two way fluid flow check valve controlled by a piston within the valve housing. Internal and external pressures move the piston towards the direction of lower fluid pressure, closing the valve. When the valve is closed, the flow of fluid from the cable portion or portions at elevations higher than the valve is prevented. Until the valve closes, however, such fluid flows out of the break. After the valve closes, fluid in the cable between the break and the valve continues to flow out of the cable through the break until there is only a small amount of fluid in the portion of the cable between the break and the valve. The cable is then subject to the risk of contamination at the break.